Flexographic printing plates are used in letterpress printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, paper, and plastic packaging films. Flexographic printing plates can be prepared from imageable elements that comprise a photosensitive layer on a substrate. The element is imaged with ultraviolet and/or visible radiation and then developed with a suitable developer leaving a printing relief, which can be used for flexographic printing. If after exposure to radiation the exposed regions of the photosensitive layer are removed in the developing process, the element is referred to as “positive working.” Conversely, if the developing process removes the unexposed regions and the exposed regions remain, the element is “negative working.” Elements useful as flexographic printing plate precursors are typically negative working, but need not be so.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions of the photosensitive layer under the clear regions of the mask but does not occur in the regions of the photosensitive layer under the opaque regions of the mask. For negative working systems, the mask is usually a photographic negative of the desired image. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the mask may change slightly in dimension due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of printing plate precursors, which obviates the need for exposure through a mask, is becoming increasingly important in the printing industry. In these processes, a computer controlled laser scans and images the photosensitive layer of the printing plate precursor. However, it has not typically been practical to use lasers to image flexographic printing plate precursors, which have relatively thick photosensitive layers. These elements have low photosensitivity and require long exposure times even with high-powered lasers. In addition, most of the imageable materials used in these elements have their greatest sensitivity in the ultraviolet region of the spectrum. Although ultraviolet emitting lasers are known, economical and reliable ultraviolet lasers with high power are not readily available. However, relatively inexpensive infrared lasers that have a useful power output are readily available.
Flexographic printing plate precursors that comprise a layer that is ablatable by infrared radiation have been used to form an integral mask over the photosensitive layer, thereby providing the advantages of direct digital imaging. The masking layer is imaged with an infrared laser and the resulting element exposed with ultraviolet and/or visible radiation through the mask. However, because ablation produces debris, the filmsetter used to image ablative masks requires additional filtration systems to prevent the debris from contaminating the optics of the filmsetter. In addition, some of the ablatable layers require large amounts of expensive infrared absorbers.
Thus, a need exists for imageable elements useful as flexographic printing plate precursors that provide the advantages of direct digital imaging but do not exhibit some or all of the disadvantages of laser exposure suffered by existing methods.